A self-propelled robot is disclosed for movement over a surface to be treated. The robot has a power supply and a pair of wheels driven by motors for moving the robot over the surface. A mechanism is provided for controllably depositing a fluent material on to the surface. Navigation sensors provide signals for enabling the robot to navigate over the surface and one or more detectors detect the presence of the material on the surface and provide signals indicative of its presence. A control system receives the signals from the sensors and detectors and controls the motors and the depositing mechanism in dependence upon the signals received from the sensors and detectors.